1. Field of the Invention
The present invention relates to apparatus for generating pseudo random codes at very high speeds. More particularly, the present invention relates to the generation of high speed composite pseudo random codes.
2. Description of the Prior Art
Heretofore, it was known that a plurality of pseudo random (PN) codes could be combined in a manner which would provide a composite PN code whose epoch length or code period is the product of the length of the individual PN codes being combined (see our U.S. Pat. No. 4,225,935). Composite PN codes provided by this system are desirable from the standpoint of acquisition because it is only necessary that the sum of the lengths of the individual codes comprising the composite code be searched. Any person not knowing the specific individual codes must operate on the composite code as if it were a very long epoch individual code. Thus, an unfriendly person not knowing the individual codes would be greatly hindered in acquiring the signal being transmitted.
It is also known in the field of communications that the higher the speed of the code sequence (chip rate), the less likely that the transmitted signal can be jammed and/or intercepted. Thus, it is desirable to transmit at extremely high chip rates to discourage jamming and/or interception. Heretofore, PN code generators have been employed to generate individual PN codes and such code generators have been the limiting factor in the speed of code generation or chip rates. A known high speed PN generator comprises a plurality of D-type flip-flops arranged in a series feedback circuit and having at least one EXCLUSIVE OR gate in the series chain. The speed of such prior art PN code generators have been limited by the type of the semiconductor device technology being employed. As semiconductor device technology improves, the speed of such prior art PN generators can also be increased. Presently, ECL semiconductor devices are representative of fastest commercially available flip-flops.
As the speeds of code generation begin to exceed 80 megahertz frequency rates or chip rates, the interconnect paths of the PN generators approach one quarter wavelength and begin to act as if they were radiating antennas or transmission lines. Such radiating transmission lines create critical problems in establishing the path lengths, in shielding and in impedance matching.
Accordingly, it would be desirable to provide a high speed PN code sequence generator capable of operating at speeds in excess of 80 megahertz without introducing problems of radiating transmission lines in the interconnecting paths of the circuitry and other high speed problems.